1. Field of the Invention
The present invention relates to an air-conditioning system for a vehicle, and, more particularly, to an air-conditioning system suitable for usage of a refrigerant having a small global warming potential.
2. Description of the Related Art
Various kinds of measure to guarantee safety of vehicles are proposed for air-conditioning systems. One air-conditioning system equipped with such safety measure is disclosed in, for example, Japanese Utility Model Publication No. S56-40182/1981. The known safety measure has a safety valve disposed in a high-pressure portion of a refrigerant circulation passage. The safety valve is designed in such a way that when the pressure in the high-pressure portion rises abnormally, its valve element is lifted from the valve sheet therefor to open the valve, thereby discharging the refrigerant from the high-pressure portion, and the fuse circuit of the compressor is opened through the valve element, thereby cutting off the power supply to the compressor.
In consideration of the global environment, air-conditioning systems which use refrigerants having a smaller global warming potential than R134a that is a conventional refrigerant have been developed in recent years. Specifically, refrigerants of this type include a new substitute flon gas like R152a and a non-flon gas like CO2 or HC. However, the use of such a refrigerant in the known air-conditioning system discussed above raises the following problems.
If a vehicle has a head-on collision and a part of the known air-conditioning system is damaged, the pressure in the high-pressure portion never rise abnormally, so that the safety valve does not open. In a case where the damaged part of the air-conditioning system lies in the refrigerant circulation passage in the instrument space therefor between the engine room and the vehicle compartment, therefore, the refrigerant which gradually leaks out from the damaged part is likely to flow into the vehicle compartment. Because the refrigerant gas such as R152a or HC is flammable, the flow of this type of gas into the vehicle compartment should be avoided. In addition, an increase in the concentration of the refrigerant, even the CO2 gas, in the vehicle compartment is not desirable from the viewpoint of safety of users therein.